, and it drove upticks in heat-related emergency visits and deaths in various U.S. states.

Hotter weather is expected to pose dangerous health risks for millions, and pregnant people are especially vulnerable to its impacts, experts caution.

"Pregnant people are physiologically more vulnerable to heat," Dr. Susanna Esposito, a pediatrician at the University of Parma in Italy, told Live Science. "Climate adaptation should treat pregnant people as a priority population."

Extreme heat is linked to adverse birth outcomes that can be fatal for both mothers and newborns. Heat can also hinder fetuses' development, contributing to long-term health and cognitive effects.

Scientists and public health experts are working to identify the thresholds between safe and dangerous heat for maternal and prenatal health. As the planet warms, how will that affect the risks associated with pregnancy? And how can those risks be countered?

Low birth weight

When the body tries to cool down, blood moves to the extremities to dissipate heat through the skin's surface. Some studies suggest that, in a pregnant person, this may reduce blood flow to the placenta, which in turn reduces the organ's supply of oxygen and nutrients. This is thought to contribute to low birth weight, which is associated with a heightened risk of illness and death shortly after birth, as well as cognitive and mental-health challenges later in life.

Heat stress may also spike the body's levels of cortisol beyond what they should be in a healthy pregnancy, studies in lab animals suggest. This may expose developing fetuses to more cortisol than usual, affecting some aspects of development and functions of the placenta. This mechanism is not fully understood in humans yet, though.

That said, several studies in humans support the notion of a link between heat exposure and low birth weight. Mothers who were pregnant during a 2003 heat wave in France were more than twice as likely to have a baby of low birth weight than mothers whose pregnancies did not coincide with the extreme heat. The trend was especially strong when the heat wave hit during a person's first trimester.

In a study conducted in Arizona, hotter temperatures during the third trimester were also associated with lower birth weight, said study co-author Kristin Mickelson, a sociologist who studies public health at the Arizona State University.

Preterm and early birth

Preterm delivery — before 37 weeks of gestation — is one of the most concerning health outcomes that's been linked to heat, Esposito told Live Science. The risk of neurological and developmental disabilities is higher in preterm babies than in full-term babies.

"If you're born preterm, you have a heightened risk of all sorts of adversities throughout infancy, but then also extending into childhood and even longer term," said Clara Heil, health researcher at Lund University in Sweden.

Severe heat may make preterm birth more likely. A review of 70 studies suggests that the chance of preterm birth is 16% higher when a multiday heat wave hits during the final weeks of pregnancy. Regarding average temperatures across an entire pregnancy, each additional 1.8 degrees Fahrenheit (1 degree Celsius) boosts the risk of preterm birth by 5%, the same review found.

High temperatures have also been tied to early births, which occur after 37 weeks of pregnancy but before 39 completed weeks. Early birth is associated with an increased risk of hospitalizations through childhood and slower neurologic development.

A study analyzing 56 million U.S. births across two decades suggested there was about 1 more early birth per 100,000 pregnancies on days with a maximum temperature above 90 F (32.2 C), compared to 60 to 70 F (15 to 21 C). Deliveries on hot days were typically about six days earlier than anticipated, but some were up to two weeks early.

With average temperatures continuing to rise, early and premature births could become more common. The United Nations predicts that the world is set to cross the 1.5 C (2.7 F) threshold of warming compared with pre-industrial levels, likely within the next five years, and we may even surpass the 2 C (3.6 F) threshold in that timeframe.

"We can probably expect to see these temperatures, or worse, in the upcoming years," Lizbeth Burgos Ochoa, statistician at Tilburg University in the Netherlands, told Live Science.

Preeclampsia, heart defects and other risks

Maternal heat exposure has also been associated with an increased risk of preeclampsia, a pregnancy complication related to blood pressure that can be fatal. In a 2011 study conducted in France, hotter temperatures during the first trimester were associated with a slightly increased risk of severe preeclampsia, although more research is needed to understand why.

Heat exposure has also been linked to other conditions of pregnancy, including gestational diabetes and hypertension, as well as increased emergency hospital admissions during pregnancy, overall.

Additional studies have highlighted potential risks for developing fetuses and newborns. For example, following a heat wave in Canada, the risk of a baby being born with critical heart defects was 6% higher, one study found. Babies with the highest risk had been exposed to temperatures of 86 F (30 C) or higher for at least 15 days during the first trimester of pregnancy.

Extreme heat waves may also raise the chance of stillbirth, according to a study conducted across six U.S. states. Four consecutive unusually hot days — exceeding the 97.5th percentile of temperature for a given county — in the last week of pregnancy was linked to a 3% increase in the risk of stillbirth.

These studies are observational, so they show links between heat and pregnancy outcomes but cannot directly show that the heat spurred the increased risks. They also don't take into account study participants' home environments or strategies for cooling off, so they don't provide a full picture of how much heat they were exposed to.

Caring for pregnant people and the next generation

Researchers are still working to understand what temperatures pregnant people should be most concerned about.

"It would be [helpful] to understand what level of heat becomes dangerous to women," Heil said. "Is there a threshold where it becomes difficult for the body to dissipate heat? And especially for the pregnant body, which is different from the non-pregnant body, to cope with that heat?"

Heil and her research team synthesized over 100 studies on heat and poor birth outcomes, such as stillbirth, preterm birth, placental abruption and preeclampsia. The studies varied considerably in that they covered different timeframes, focusing on just one trimester or all three, for instance. But in general, their results converged on a pattern: The risk of poor outcomes rose as temperatures hit about 87 F (30.5 C). At 102 F (39 C), the combined risk of poor outcomes rose by 20%.

This summer, daytime maximum temperatures in the continental U.S. averaged a record-breaking 88.6 F (31.4 C). The number of days per year over 105 F (35 C) is expected to triple in the U.S. by midcentury.

Heat's detrimental effects on birth outcomes are even more extreme for those with disadvantaged socioeconomic conditions, Burgos Ochoa noted. People with lower incomes are less likely to have air conditioning, and neighborhoods with a lower median income tend to have less green space, which contributes to trapped heat in their immediate surroundings. Expanding green spaces and adding bodies of water can help cool down neighborhoods and mitigate heat impacts on health, she said.

Heil's research team has developed a phone app for pregnant people and those who have recently given birth that forecasts the risk of heat stress and suggests actions to take to mitigate those risks, such as hydrating or changing clothing. Heil is hopeful that the app and similar early-warning systems can make a difference.

"Maternal health should be at the top [of our concerns], because it is about the next generation," Mickelson said. "If we don't take care of the mothers that are giving birth to the next generation, that's going to have long-term consequences down the road."

This article is for informational purposes only and is not meant to offer medical advice.

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'> Rising temperatures could boost pregnancy risks worldwide — here's how

Glaucoma, which affects about 80 million people worldwide, is a group of diseases that damage the optic nerve, which carries visual information from the eye to the brain. It can lead to permanent vision loss, usually affecting peripheral vision at first and later central vision. Cataracts are even more common, affecting more than 94 million people globally; the condition occurs when the eye's lens becomes cloudy, causing blurred vision and, eventually, blindness.

Aging raises the risk of both diseases, as do certain medical conditions, such as diabetes. But now, a review of 41 studies suggests that people who are exposed to higher levels of air pollution may also face a greater risk of these diseases. The research, presented in September at the 44th Congress of the European Society of Cataract and Refractive Surgeons in London, brought together evidence from dozens of studies collectively involving millions of people across East Asia and the U.K.

The studies were observational, meaning they can only draw a link between air pollution and the eye conditions, rather than prove that air pollution causes the diseases. But still, "this reframes glaucoma and cataract as conditions shaped by where you live and breathe, not just how long you've lived," said study co-author Dr. Ahmed Alnabihi, an ophthalmologist at King Khaled Eye Specialist Hospital and Research Center in Saudi Arabia, who presented the findings. These environmental exposures may compound age-related decline in the eye as they do in cardiovascular disease, he said.

PM2.5 shows the clearest links

The researchers examined exposure to several pollutants, including different sizes of particulate matter (PM), such as fine PM2.5 and larger PM10. They also looked at nitrogen dioxide, sulfur dioxide, ozone and carbon monoxide.

Most studies included in the review estimated people's long-term exposure based on the pollution levels around their homes and in their communities. They used a range of methods, including government monitoring systems, satellite data and computer models designed to estimate pollution levels. A few studies examined short-term exposure and its relation to more sudden eye issues, such as acute angle-closure glaucoma, a medical emergency that can quickly cause vision loss.

Using this wealth of data, the team compared pollution exposure with rates of glaucoma, cataracts and age-related macular degeneration (AMD), another common eye condition that impairs vision. PM2.5 — tiny particles that measure 2.5 micrometers or smaller — showed the clearest associations with glaucoma and cataract risk.

People with the highest exposure to PM2.5 had 70% higher odds of glaucoma, compared to those with the least exposure. Plus, an increase in PM2.5 exposure equivalent to the difference between the 25th and 75th percentiles of exposure within each study was associated with 8% higher odds of glaucoma and a 4% higher hazard of cataracts.

The evidence was less consistent for air pollution and age-related macular degeneration, which damages the central part of the retina that's responsible for sharp, detailed vision. And unlike the clear connection seen with PM2.5, other pollutants showed less consistent links to eye disease. For instance, nitrogen dioxide was not significantly tied to AMD, ozone was not tied to cataracts, and neither pollutant was linked to glaucoma. Sulfur dioxide and PM10 showed no significant associations with any of the three conditions.

The association between PM2.5 and glaucoma was relatively consistent across the reviewed studies, said Nidhi Singh, an environmental epidemiologist at the IUF-Leibniz Research Institute for Environmental Medicine in Germany, who was not involved in the study. The findings for PM2.5 and cataracts were moderately consistent, albeit more variable than the glaucoma data; still, she said that the analysis seems to support a link.

Singh did caution that the underlying evidence is observational. "Therefore, the results demonstrate an association, not by themselves a causal effect of PM2.5 on glaucoma or cataract," she told Live Science.

What explains the increased risk?

The exact biological mechanisms behind these links remain unclear, but Alnabihi noted that fine particulate matter is small enough to be absorbed into the body. This can trigger oxidative stress, in which harmful molecules overwhelm natural defenses and cause damage. It also drives chronic, low-grade inflammation, in which the immune system stays mildly activated for long periods, he explained.

In the eye's lens, these processes can "damage the crystallin proteins that keep it transparent, causing them to misfold and cloud, which is what a cataract is," Alnabihi said.

In the optic nerve, the same processes are thought to damage retinal ganglion cells, a special type of neuron that relays information from the retina to the brain. This can contribute to glaucoma independent of internal eye pressure, he said, which is another big risk factor.

"Unlike the lungs or gut, its [the eye's] surface is in direct, continuous contact with outdoor air, giving pollutants a uniquely direct route that might cause damage," he added.

Other studies have found clues that support the link between PM2.5 and eye damage. Mice that were exposed to ambient PM2.5 for nine months developed elevated pressure inside their eyes, a glaucoma risk factor. The researchers observed both oxidative stress and nitrative stress — a similar phenomenon — in the tissues that drain fluid from the eye, which may help to explain the rise in pressure.

Studies of humans and human cells have uncovered similar patterns. In data from more than 480,000 people in the UK Biobank, a large repository of health information, those exposed to higher PM2.5 were more likely to develop glaucoma over time. In another study, scientists exposed lab-grown human retinal tissue made from stem cells to PM2.5. The particles disrupted retinal cell development, including that of retinal ganglion cells, while increasing cell death.

These studies offer pieces of a possible mechanism, but for now, how those pieces fit together in people remains unclear.

This article is for informational purposes only and is not meant to offer medical advice.

'> 'A uniquely direct route that might cause damage': Scientists uncover link between air pollution and eye diseases like glaucoma begin to recover until the 16th century.

The Black Death era is the perfect opportunity to look at the effects of humans on biodiversity over a longer period, Jonathan Gordon, a paleoecologist at the Leverhulme Centre for Anthropocene Biodiversity at the University of York and first author of the paper, told Live Science.

In the study, which was published March 4 in the journal Ecology Letters, the researchers compared more than 100 European pollen records that included about 4,600 samples. The records came from sites ranging from the lowlands of France and uplands of the southwest United Kingdom to the Swiss Alps.

The pollen records "give us information on the past diversity of the plant records," Gordon said.

For example, cereal grains ‪—‬ such as wheat, barley and rye ‪—‬ were a fundamental part of medieval agriculture. The representation of pollen from these types of crops in the pollen record points to historic land use.

The researchers divided the pollen data into three categories based on the presence of cereal pollen: places where agricultural production went up, places where it went down, and places where there was no evidence of cereal cultivation.

"We found a decline in biodiversity overall, but the decline overall was driven by those landscapes that declined in agricultural production," Gordon said.

"In those landscapes where agricultural production went down, there was a reduction in biodiversity of around 40%. Then, after about 150 years, as populations started recovering and landscape disturbances started increasing again, biodiversity went up to slightly above the level it was before."

Mosaic landscapes

Many people think there was a "stable and pristine world in which the entry of people has only one result: net loss of species and homogenization," Erle Ellis, an agroecologist at the University of Maryland, Baltimore County who was not involved in the research, told Live Science in an email. "The paleoecologists have always known the story is far more complicated."

There is a growing body of evidence showing that people's long-term cultural shaping of their landscapes often creates more diverse habitats, known as "mosaics," that can sustain more species than simple homogenous landscapes can, he said.

Whereas modern agriculture focuses on single crops on plots of land, in previous centuries, "you had all these different types of agriculture operating within the same landscape," Gordon said.

"In that landscape, you had lots of natural features that remained from the Middle or Early Holocene (which began after the last Ice Age about 12,000 years ago)," he added. "You had old bits of woodland, you had heaths, you had ponds, and bits of secondary forest. It was all interlaced with these agricultural mosaics, and that led to raised biodiversity because there were more habitats for different plant species to exploit."

The findings highlight the differences between medieval and modern land use practices — and their consequences for biodiversity. Research suggests that during the 20th century, global biodiversity declined by up to as much as 11% and human activities‬, such as land-use changes, pollution, and climate change‬, are the major culprits. In agriculture, the practice of monoculture — where a single crop species is grown over a large area — reduces biodiversity.

"What this work contributes to showing is that, until relatively recently, even in Europe, very different forms of land management and interaction were practiced than exist today," Patrick Roberts, director of Department of Coevolution of Land Use and Urbanisation at the Max Planck Institute of Geoanthropology, told Live Science in an email.

"Rather than the monoculture, plantations, or uniform changes to landscapes we see in the 20th and 21st centuries, emphasis was placed on maintaining 'mosaics,'" said Roberts, who was not involved in the new research.

In the study, "we can see how relationships between people and biodiversity have changed through time and how people can actively use landscapes while maintaining biodiversity and mixed ecosystem functions," he said.

There is still more to understand about how humans' disappearance from landscapes in the Black Death era affected the natural world, Gordon said. "There are a lot of other types of diversity ‪—‬ not just the number of types, but what the different types present actually do." There may be a hundred types of plants in a landscape, but if they do the same thing, "you have low functional diversity," he said.

Functional diversity "is super important for how ecosystems adapt to changes in climate and how resilient they are, which is something we need for future-proofing our agricultural system," Gordon said.

'> Black Death had dramatic effect on species diversity as humans abandoned farmland, study finds Timothy Cunningham, an assistant professor of astronomy and astrophysics at the University of Warwick who led an international team of researchers to study the object, said in a statement.

SN 1181 is one of only five supernovas in the Milky Way galaxy known to have been witnessed from Earth before the invention of the telescope. (Those optical instruments first appeared in the Netherlands in 1608.) Now, advanced ground-based telescopes can produce wonderfully detailed images of those historic remnants. This latest image is from the Gemini North telescope on Mauna Kea in Hawaii and reveals about 10 times more filamentary structures in Pa 30 than have been seen before. This extraordinary detail gives astronomers fresh clues about the unusual explosion and the environment surrounding the star.

"These images give us the clearest picture yet of Pa 30's structure, and it's not what we expected," said Cunningham, whose team published their findings in The Astrophysical Journal. "The filaments are made of chains of tiny knots rather than smooth streaks, providing new constraints on the nature of the space around the star before it exploded and the physical processes that have shaped the expanding ejecta."

The knot-like structures may have resulted from variations in temperature or density in material surrounding the star as debris from the explosion expanded.

Pa 30 contains another unusual feature: a surviving star that sits almost exactly at the center of the remnant. That's highly unusual, because a supernova explosion typically sends stars away from their original positions. It suggests that the original explosion was highly symmetrical, the researchers said.

Furthermore, Pa 30 is thought to be a Type Iax supernova. Unlike standard Type Ia supernovas, which destroy their stars, a Type Iax event is a lower-energy, incomplete explosion of a white dwarf star that can leave behind a remnant known as a "zombie star." It's the only Type Iax supernova ever found.

'> This 850-year-old 'firework' supernova is full of mysterious knots

A calculated hunter

Cuttlefish are visual hunters, using their large, complex eyes to locate and track potential meals. In general, their strategy involves a slow, stealthy approach, said Bret Grasse, the manager of cephalopod operations at the Marine Biological Laboratory in Woods Hole Massachusetts.

"The cuttlefish will slowly approach the prey so as to not alert the unsuspecting shrimp, fish or crab," Grasse told Live Science in an email.

During their approach, cuttlefish often put on a showy display, producing moving bands across their body. These displays are produced by a combination of light-reflecting pigment cells known as iridophores, leucophores and chromatophores. These cell types contract and expand to produce specific colors and patterns, all within milliseconds.

Some may wonder whether this trick can mesmerize prey, but scientists don't think that's the case.

Hypnosis, defined as a mental state of increased relaxation and altered awareness, is thought to be a uniquely human psychological state. There is very little research on whether fish can be hypnotized, but a 2005 study found that pushing a steady stream of water through the snapper (Chrysophrys auratus) put them in an immobile state similar to being hypnotized. Another study looked at sedative hypnotic drugs like benzodiazepines and barbiturates and their effects on zebrafish, finding that the fish had molecular receptors for these drugs, which helped render them immobile. However, whether fish can be visually put into a state like hypnosis has yet to be directly studied.

And while some animals, such as chickens, can be induced into trance-like conditions of tonic immobility, an involuntary, temporary state of unresponsiveness that happens when humans or animals experience extreme trauma, it's unlikely that the cuttlefish's color changes "hypnotize" their prey, Connor Gibbons, the cephalopod facility manager at Columbia University told Live Science. Instead, some researchers believe that the color show serves as an ingenious disguise.

As waves of different colors ripple down the cuttlefish's body, the shape of the animal becomes less obvious to its prey, so they may not see the cuttlefish as a threat to flee from, Gibbons said.

"It's only part of something looming at them, which they're like, 'I don't know if you are a predator? Are you just passing waves? What are you?'" Gibbons said. "And then by that time, it's too late."

Once they get close enough, the cuttlefish shoots their two long, specialized feeding tentacles from small hidden pouches near the animal's mouth at their prey.

"The closest thing I can compare it to is the tongue of a chameleon extending out to grab an unsuspecting insect," Grasse said. "Cuttlefish feeding tentacles will fire out in the blink of an eye before the prey can react and escape."

Then, the prey is pulled toward the cuttlefish's mouth, pierced by its sharp, chitinous beak and injected with venom. The use of the venom helps the cuttlefish reduce their energy usage when hunting "as the cuttlefish does not want to use unnecessary energy and struggle against the prey while it's trying to eat," Grasse said.

However, the notion that cuttlefish flash colors to hide their body shape is only one idea of why cuttlefish perform these strobing color displays. Other researchers, including Grasse, describe them as a way to distract their prey.

"Essentially, they are trying their best not to alarm the prey to their presence," he said. By breaking up the size of their bodies, cuttlefish can convince their prey they're not as large or alarming as they may seem.

Whether the light shows are for disguise or distraction still needs parsing out, Gibbons said, and scientists also need a better understanding of how the cuttlefish controls its deadly dermal display. "The jury is still out on exactly what is happening," he said.

How much do you know about octopuses? Find out with our octopus quiz!

'> Can cuttlefish hypnotize their prey?